The present invention relates generally to mechanical lifting apparatus, and particularly to load balancing arms for carrying the weight of a load and allowing an operator to then move the load by operator applied forces within a work space.
A load balancing arm assists an operator in manipulating a load within a given work space. The load balancing arm attaches to and carries the weight of the load by maintaining a constant lifting force on the load throughout the work space. The operator manipulates heavy loads, much greater than the operator's lifting ability, as if the loads weighed little or nothing. In other words, the operator is not required to move the load in the fashion of a button controlled hoist whereby, for example, the operator presses an "up" button to move the load up and a "down" button to lower the load. In contrast, load balancing arms enable the operator to grab and move the load directly in a natural fashion as if the weight of the load was within the operator's lifting capacity and without manually operating control buttons to control direction of movement.
A load balancing arm is also useful for workers doing moderately strenuous repetitive tasks such as transferring items between a bin and a conveyor. Even if the weight of one item is well within the worker's lifting capability, because the worker repeats the task all day, the job can be strenuous. Because the load balancing arm carries the weight of the load, the job is less strenuous and, therefore, more productive. As may be appreciated, however, in performing such repetitive tasks, it is desirable that the load balancing arm allow natural movement of the load and not resist operator applied forces.
Unfortunately, movement of a load within the work space requires operator applied forces to the load balancing arm or the load itself in order to accelerate the load into motion. The operator must overcome certain counteracting forces present in the system, i.e., forces reactive to operator applied forces, before the load will move. These reactive forces are generally a function of the weight of the load and can affect significantly the use of a load balancing arm for repetitive tasks with moderately heavy loads, e.g., on the order of pounds and less frequent tasks with very heavy loads, e.g., on the order of pounds.
The inertia of the load resists movement as does all mass accelerated into motion by application of force. The greater the weight of the load, the greater the mass and, therefore, the greater the resistance to operator applied force.
Frictional forces within the structure of the load balancing arm also resist movement of the load by operator applied force. These reactive forces can vary depending on mechanical design, but some friction will always be present in the structure of a load balancing arm. Most load balancing arms use a linkage assembly as an arm structure and friction exists between these links. Before the load can move, the operator must apply sufficient force to overcome friction within the links. The greater the lifting capacity for the load balancing arm the greater the mechanical friction and, therefore, the greater the resistance to operator applied force.
The active components of the system responsible for maintaining a constant lifting force on the load also resist operator applied forces. Pneumatic cylinders are used to maintain a constant force against one side of a lever arm within the load balancing arm in order to maintain a constant lifting force on the load. When the operator moves the load, however, this changes the volume of the air chamber within the cylinder and, therefore, changes the pressure in chamber. As a result the lifting force deviates from the desired lifting force. Air pressure regulators have been used to maintain a given pressure set point within the pneumatic cylinder, but these regulators have introduced counteractive hysteresis into the system. Before the air regulator can detect a change in pressure and adjust the lifting force applied to the load, the operator must actually move the load to affect air pressure within the pneumatic cylinder. Furthermore, the air pressure regulator cannot react immediately, having neutral band above and below the pressure set point where it makes no pressure correction, and the operator must move the load through sufficient range of motion to develop the necessary pressure differential to actuate the air regulator. For heavy load capacity systems, the greater the load and, typically, the greater the counteractive hysteresis.
It is, therefore, desirable to reduce such forces counteractive to operator applied forces.
In U.S. Pat. No. 3,880,393 issued Apr. 29, 1975 to Robert W. Watson and entitled Load Balancer With Balance Override Control, a load balancing arm is shown and described as including an override control for active movement of a load. The override control takes the system out of a balanced condition, bypassing the normal control circuitry and imposing a control pressure different from that utilized in maintaining the load in a balanced condition. An operator must separately actuate the mechanism to urge the load in a desired direction of movement. The Watson Load Balancer, therefore, does not provide the type of "natural" manipulation of the load as is desired in a load balancing arm. In other words, the Watson machine simply has a "hoist mode" where the override control drives the load into motion by adjusting the lifting force applied to the load. Thus, the Watson device does not provide for "hands on" movement of the load as if the operator were really picking up the load, rather the operator must operate a separate control button in the fashion of a hoist control system.
U.S. Pat. No. 3,721,416 issued Mar. 20, 1973 to N. G. Goudreau and entitled Loading Balancer shows a load balanced arm which responds to an operator applied force to then urge the load in a corresponding direction. The loading balancer shown by Goudreau, however, does not solve the problem of eliminating or reducing forces counteractive to operator applied force. In the Goudreau device, the operator must still exert a given magnitude of applied force in order to first move the load before the system can implement a change in the lifting force. In the Goudreau device, a "feeler" cylinder responds to movement of the load to detect an indicated direction of movement and then biases the lifting force in order to accomplish that direction of movement. Important to note, the operator must overcome the inertia of the load, the friction within the system, and any hysteresis present in the system before the system can accomplish the task of actively moving the load in the desired direction.
In use of a load balancing arm, the operator moves a load engaging mechanism, located at the distal end of the load balancing arm, next to the load. The operator actuates the load engaging mechanism and it attaches to the load in some fashion, e.g., such as by clamping or vacuum. The load balancing arm, once coupled to the load, introduces the lifting force to accept at least some of the weight of the load. A lifting force greater than the weight of the load is undesirable because the arm would travel with the load unless held back by the operator. In such case the operator expends energy, i.e., operator applied force, even with the load stationary within the work space. A lifting force less than the weight of the load also allows the load to travel and always requires some effort by the operator in carrying the remaining weight of the load, whether the load is held stationary or moved through the work space. As may be appreciated, the closer the load balancing arm lifting force is to the weight of the load, the less operator applied force is necessary to move the load freely through the work space. The lifting force may vary somewhat from an ideal balance condition, i.e., a lifting force matching exactly the weight of the load, to the extent that friction and system hysteresis holds the load against travel. The goal has been to provide a lifting force substantially equal to the weight of the load, or at least close enough to prevent travel of the load, and allow the worker to move the load within the work space by simply overcoming the inertia of the load, friction within the arm and any hysteresis of the components providing the constant load balancing lifting force.
If the load engaging mechanism loses the load while the load balancing arm is applying a lifting force, the load balancing arm can suddenly "fly-away" as it attempts to maintain a constant lifting force. Thus, an important safety aspect of load balancing arms relates to detection and appropriate response to a lost load condition. Current lock-up safety systems use hydraulic type systems to prevent "fly-away." The hydraulic systems introduce additional resistance to easy movement of the load by increasing friction, i.e., friction within the hydraulic system added, and hysteresis, the force needed to move the hydraulic fluid within the safety lock-up system.
Load balancing arms typically include a pantographic arm. A pantographic arm generally comprises a vertically rotatable housing carrying a pantographic arm linkage with a load engaging mechanism at the distal end of the arm. The pantographic arm is characterized by a pivotally mounted lifting arm attached to the vertically rotatable housing and allowed rotation about a fulcrum. A counter weight on one side of the fulcrum counter balances the weight of the lifting arm. A downward extending work arm pivotally attached on the other side of the fulcrum to the distal end of the lifting arm extends into the work space and carries at its distal end a load engaging mechanism, typically specific to the type of load to be manipulated. Third and fourth links of the pantographic arm pivotally couple to one another and to the lifting arm and work arm, respectively, to complete a parallelogram linkage.
The above-noted U.S. Pat. Nos. 3,880,393 and 3,721,416 each show such pantographic load balancing arms. Other examples of load balancing arms generally characterized as pantographic arms may be found in the following U.S. Patent Documents: U.S. Pat. No. 4,666,364 issued May 19, 1987 to Doege, et al and entitled Low Friction Cylinder For Manipulators, based on the pantograph principal and equipped with a pneumatic balancer control; U.S. Pat. No. 4,659,278 issued Apr. 21, 1987 to Doege, et al and entitled Manipulator, based on the pantograph principal; U.S. Pat. No. 4,421,450 issued Dec. 20, 1983 to Kouno and entitled Cargo Handling Apparatus; U.S. Pat. No. 4,215,972 issued Aug. 5, 1980 to Yamasaki, et al and entitled Transfer Mechanism Employing Swingable Arm Formed As A Parallelogram Linkage; U.S. Pat. No. 3,883,105 issued May 13, 1975 to Matsumoto and entitled Load Handling Equipment; U.S. Pat. No. 3,747,886 issued Jul. 24, 1973 to Carlson, et al and entitled Load Balancer With Safety Control; U.S. Pat. No. 3,615,067 issued Oct. 26, 1971 to N. G. Goudreau and entitled Load Balancer; U.S. Pat. No. 3,259,351 issued Jul. 5, 1966 to R. A. Olsen and entitled Load Balancer Assembly; and U.S. Pat. No. 3,259,352 issued Jul. 5, 1966 to R. A. Olsen and entitled Loading Balancer Assembly. A review of the above-noted U.S. Patents provides a history of pantographic arm development and is believed representative of the type of arm linkages used in such pantographic arm arrangements for load balancing arms.
The load engaging mechanism can comprise suction cups held against a package and an operator actuated button for drawing air from under the suction cups to produce a vacuum between the load engaging mechanism and the package. Other examples include clamping jaws responsive to an operator actuated button to close and grasp the load. While many load engaging mechanisms would include an operator actuated button for actuating the load engaging mechanism, such a load engaging mechanism could be automatically actuated when brought next to the load. Many load engaging mechanisms have been provided at the distal end of load balancing arms.
The pantographic parallelogram linkage provided by the arm maintains the load within a horizontal plane when operator applied forces are horizontal. In other words, due to the pantographic geometry of the arm the operator may apply strictly horizontal forces and move the load along a horizontal plane within the work space.
Load balancing arms include some mechanism for applying the lifting force to counter balance at least part of the weight of a load attached to the distal end of the arm. Such lifting force has been applied to the pivotally mounted lifting arm of the pantographic parallelogram linkage to provide an upward force on the load, i.e., rotate the lifting arm about its fulcrum in suitable direction to lift the load.
Other examples of load balancing arms take a variety of forms, but may be generally characterized as devices which carry the weight of a load and allow an operator to then move the load freely throughout a work space. In such load balancing systems, the operator applies forces directly to the load or the load balancing arm in order to move the load. In such load balancing systems, operator applied forces must overcome the inertia of a load, friction within the system, and hysteresis within the active components of the load balancing arm which maintain a constant lifting force on the load throughout the work space.
Load balancing arms according to current practice are efficient at handling consistent weight loads repeatedly. The lifting force required to balance the unloaded and loaded conditions can be preset at the regulator and the regulator may be switched between two such lifting forces in use of the load balancing arm as it switches between its unloaded and loaded conditions. When balancing multiple weights is required, however, cumbersome and slow automatic or semi-automatic systems are currently used, but do not provide the type of flexibility in available lifting forces which would be desirable, especially in the context of loads having varying or unpredictable weights. When the operator needs to move the load, the operator must accelerate the load in some fashion and overcome the inertia, friction and hysteresis of the system. Thus, load balancing systems according to current practice tend to exert counteracting forces responsive to operator applied forces, and therefore tend to make the task of moving a load more difficult.